1. Field of the Invention
The present invention relates generally to methods for separating fine powder or particles of different densities and more specifically to a method for separating of such particles using the levitational properties of a ferrofluid.
2. Description of the Invention
There exist standard methods of separating two or more solids which depend on the differences in the densities of the components. The most common method involves sink-float separation in a liquid medium. Sink-float separation operates on the principle that when two objects of different density are immersed in a fluid of intermediate density, the less dense will float and the more dense will sink. The separation is completed by individually removing the two solid fractions. While simple in principle, solids separation by the classical sink-float method has a number of severe shortcomings. These include:
A. Pure liquids or solutions do not cover the range of densities of interest. Most liquids have low densities whereas most solids have high densities. There are very few materials that are liquid at ambient temperature and exhibit a density greater than 2 gr/cm.sup.3. PA1 B. An accurate sink-float separation is obtained only if there is complete liberation of the different particles in the solid mixture. It is necessary to ensure that a sample of very fine particles is well dispersed in the sink-float medium. Otherwise, agglomerates will behave as individual particles with a density intermediate between those of the ultimate particles in the agglomerate, thereby resulting in poor separation. PA1 C. The density of a given liquid is a constant value which is not readily varied. Changes in temperature will only result in small variations in the liquid density. In order to obtain different density cuts, it is necessary to carry out a series of sink-float separations in different liquids of varying densities.
Use of a ferrofluid and a magnetic field to create an apparent high ferrofluid density permits sink-float separation of non-magnetic solids according to their density. Some of the problems outlined above are resolved by the controlled density aspect of ferrofluids.
Ferrofluids are stable colloidal dispersions of superparamagnetic particles (diameter (d) .perspectiveto. 0.01 microns). These dispersions retain their liquid properties in a magnetic field. By proper choice of stabilizing agents, magnetic properties can be conferred to a wide range of liquids which include water, hydrocarbons and fluorocarbons. These colloidal dispersions form a unique class of magnetic liquids in which it is possible to induce substantial magnetic body forces. One of the unusual properties of a ferrofluid is that its apparent density may be made significantly greater than its true physical density by the application of a magnetic field. With a properly designed electromagnet, the apparent density of a ferrofluid may be varied from less than 1 gr/cm.sup.3 to more than 25 gr/cm.sup.3, thereby permitting flotation of any element in the periodic table. The concepts involved are more fully discussed in U.S. Pat. Nos. 3,483,968; 3,483,969; and 3,488,531which are commonly assigned to the assignee hereof and are incorporated herein by reference.
A variation of sink-float separation in a liquid of constant density is the classification of particles according to their densities in a density gradient column. In a density gradient column, the density of the liquid increases with depth; the liquid at the top of the pool is less dense than the liquid at the bottom of the pool. A density gradient can be established either by imposing a temperature gradient on a pure liquid, adding heat at the top of the pool and removing it at the bottom, or by having a concentration gradient. In the latter case, two miscible liquids of different density are added in variable proportions along the length of the column. The concentration of the denser liquid increases with column depth.
Density gradient classification or sink-float separation have the drawbacks alluded to above for classic sink-float separation and additional difficulties as well: (a) any mixing or turbulence destroys the density gradients, irreversibly in the case of columns based on a concentration gradient; (b) preparing and maintaining a gradient column is difficult to accomplish. Introducing and removing solid particles without upsetting the density gradient is difficult.
However, ferrofluids would seem well suited to creation of a stable variable density fluid through appropriate application of magnetic forces. Moreover, a density gradient system is superior for classifying fine particles.